Add default argument values to builtin types

[srittau]

Mark two exception name arguments as being potentially None

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pydantic (https://github.com/pydantic/pydantic)
+ pydantic/errors.py:131: error: Argument "name" to "PydanticUndefinedAnnotation" has incompatible type "str | None"; expected "str"  [arg-type]

[srittau]

The primer fallout is smaller than expected. We could use str | MaybeNone for the attributes of AttributeError and ImportError, but considering the low primer fallout, consistency with NameError (which always used str | None), and type safety I suggest we keep str | None.